The deterioration of structural materials caused by exposure to the elements continues to present a constant problem for industrialized societies. Therefore, coatings of diverse formulations, which hinder or prevent deterioration, are indispensable for the maintenance and survival of nearly every construction material used in all conceivable structures from buildings to bridges, from ships to automobiles, etc. Structural materials exposed to air face cyclical changes in temperature and moisture. If such materials are left unprotected they will rust, crack, and eventually disintegrate. In similar fashion, structural materials immersed in seawater face cyclical changes in temperature and continuous exposure to water, saline, and living organisms. Such materials will electrochemically corrode, rust, foul, crack, and eventually disintegrate without substantial protection.
From the earliest times various coatings and paints based on combining pigments with unsaturated oils have been developed. During the early years of the twentieth century, coatings based on different derivatives of cellulose were introduced to provide rapidly drying paints for the automotive industry. The mass-production requirements of the automobile industry spurred continued development of rapidly drying paints. Consequently, the cellulose coatings were followed by the rapid evolution of other synthetic polymers that offered many different coatings to protect metals from corrosion.
Nevertheless, development of corrosion inhibiting coatings that spontaneously adhere to metals and their alloys, especially when the metals or alloys are wet, was an elusive goal until recently. An answer to this problem was recently discovered with the advent of a class of poly-amine-quinone polymers (otherwise known as "PAQs") which have very strong, inherent affinity to all metals and their alloys. A more detailed description of these poly-amine-quinone polymers, their functionality, and their use are described by my U.S. Pat. Nos. 4,831,107 (Erhan); 4,882,413 (Erhan); 4,981,946 (Erhan); and, 5,284,683 (Erhan). These poly-amine-quinone polymers exhibit certain unusual characteristics including:
a) extraordinary affinity towards all metals and their alloys, strong enough to displace water from wet, rusty metal surfaces; PA1 b) total water-repellency when cured, either chemically or thermally; PA1 c) excellent anticorrosive properties; PA1 d) sufficient flexibility to allow a coated panel, cured with epoxy resins, to be bent on an 1/8" mandrel; PA1 e) very good impact and abrasion resistance; PA1 f) ability to bind to siliceous materials; PA1 g) ability to be cast as free standing films; PA1 h) solubility in suitable organic solvents to provide paints and coatings, especially anticorrosive automobile and marine paints and coatings, according to my U.S. Pat. Nos. 4,831,107 (Erhan); 4,882,413 (Erhan); and, 4,981,946 (Erhan); PA1 i) ability to metallize plastics according to my U.S. Pat. No. 5,284,683 (Erhan); PA1 j) ability to displace machine oils from metal surfaces; PA1 k) resistance to certain organic solvents, including Skydroll.RTM., in sodium hydroxide; PA1 l) resistance to short wavelength ultra-violate light; PA1 m) nonflammability; and, PA1 n) ability to conduct heat.
Several methods are known for the preparation of poly-amine-quinone polymers. One method for preparing poly-amine-quinone polymers involves the polymerization reaction of various aliphatic and aromatic polyamines with various quinones as taught by my U.S patents described hereinabove. In this method, the reaction steps include dissolving the reactant polyamine and quinone in an appropriate solvent, for instance, acetone, ethanol, or a mixture of solvents; admixing the reactant solutions in a mole ratio of polyamine to quinone preferably of about 1-2:3; refluxing the reactant solution with stirring for several hours, preferably for about 3 to 8 hours, at temperatures dictated by the chemical nature of the reactant polyamine and quinone; removing the solvent under vacuum, and recovering the poly-amine-quinone product by washing with water or solvent and drying in a vacuum oven, preferably for over 3 hours at a temperature of about 30.degree. to 70.degree. C.
While this method works for both aliphatic and aromatic polyamines and different quinones, certain problems accompany this preparation approach. First, because the polymerization requires oxidative conditions to prevail and because the reactant quinone can act as the oxidizing agent, two thirds of the quinone is wasted as the oxidant, rather than being incorporated into the polymer product. Second, when the oxidizing agent is benzoquinone, the reduction of benzoquinone during the polymerization reaction generates an equivalent quantity of hydroquinone, which can compete with the poly-amine-quinone polymer product for the binding sites on metals, thus decreasing the effectiveness of the poly-amine-quinone polymer coating. Therefore, the hydroquinone must be removed by extensive washing of the polymer, which consequently increases the cost of production.
Another method for preparing similar polymers involves substitution polymerization in m-cresol as taught in Ueda, M., Sakai, N. and Imai, Y., "Synthesis of Polyaminequinones by Vinylogous Nucleophilic Substitution Polymerization of 2,5Disubstituted p-Benzoquinones with Diamines", Makromol. Chem. 180, 2813 (1979). In this method, the starting materials are 2,5-dichloro-benzoquinone, 2,5-dihydroxy-benzoquinone, or 2,5-dimethoxy-benzoquinone and aromatic amines. This method is limited to a laboratory procedure, however, because of the cost of the precursors and the expense of removing the m-cresol from the product mixture.
A further method for preparing poly-amine-quinone polymers involves the reaction of stoichiometric ratios of polyamines and quinones with an external oxidizing agent, such as calcium hypochlorite as taught by Nithianandam, V. S. and Erhan, S., "Quinone-amine polymers: 10. Use of calcium hypochlorite in the syntheses of polyamine-quinone (PAQ) polymers", Polymer 32, 1145 (1991). This approach also has disadvantages, however. First, the quality of the oxidizing agent is critical, even a slight amount of moisture reduces the efficiency of the reaction dramatically. Second, this method cannot be used with aromatic diamines because they are preferentially oxidized causing unacceptable low yields. Third, once the reaction is complete, inorganic salts must be removed from the product, increasing the cost of manufacture.
A variety of other oxidizing agents capable of converting hydroquinone to benzoquinone are known. Most of these oxidizing agents, however, can only be used in water or aqueous organic solvents, which precludes their use with poly-amine-quinone polymers because such polymers are insoluble in water or aqueous solutions. Furthermore, some of the oxidizing agents, which can be used in organic media, were shown to oxidize the amine. Sodium chlorate in the presence of catalytic quantities of ammonium vanadate has been tried for the synthesis of poly-amine-quinone polymers, with some success, however, the reaction is excessively sensitive to variations in the reaction conditions. A method used in the preparation of simple aminoquinones from hydroquinone in aqueous medium, utilizing sodium iodate is taught in Schofer, W., Aguado, and A. Sezer, U., "Oxidative Aminierung von Hydrochinonen," Angew. Chem. 83, 441 (1971).
From an industrial standpoint both air and oxygen are ideal oxidizing agents due to their abundance and because they are relatively inexpensive. Earlier attempts to prepare poly-amine-quinone polymers with air and oxygen were generally unsuccessful. While polymer was produced, only a small fraction of the polymer was insoluble in water, suggesting that most of the product was oligomeric in nature as shown by Nithianandam, V. S. and Erhan, S., "Quinone Amine Polymers. IX. Attempts to Synthesize Polyamine-Benzoquinone Polymers Using Air and Oxygen as Oxidizing Agents", J. Appl. Polym. Science 42, 2385 (1991). On the other hand, simple bis-(dialkylamino)quinones were prepared using oxygen as taught by Baltzly, R. and Lorz, E., "The Addition of Dimethylamine to Benzoquinone", J. Am. Chem. Soc. 70, 861 (1948). Later studies, however, pointed out several major problems with this method. One such major problem was that the method could only be used with secondary amines having certain specific structural characteristics as taught by Crosby, A. H. and Leitz, R. H., "A Study of an Oxidative-amination Method for the Synthesis of Aminoquinones", J. Am. Chem. Soc. 78, 1233 (1956).
What is needed is a simple, efficient and economical method for the preparation of metal binding poly-amine-quinone polymers, which can be used, inter alia, as paints, coatings, binders, adhesives, and curing agents, without the drawbacks mentioned hereinabove.
The present invention alleviates the unfavorable or prohibitive economic concerns associated with scaling up to the level of commercial production the previously known methods of preparing poly-amine-quinone polymers. The present invention also alleviates, inter alia, the need for increased purification of the reaction solution to isolate the product poly-amine-quinone polymer, the need for tight control of the quality of the oxidizing agent used, and the need for more expensive precursors associated with certain other known methods for synthesizing poly-amine-quinone polymers. Moreover, unlike some of the previously known poly-amine-quinone polymer preparation methods, the present invention works for both aromatic diamine and aliphatic diamine derived poly-amine-quinone polymers. The present invention advantageously allows for the polymerization of quinones or quinone precursors (i.e., a quinone in reduced form), especially hydroquinone, with polyamines in stoichiometric ratios. The present invention also advantageously alleviates the waste of two thirds of the quinone as the oxidizing agent associated with previously known preparation methods.